Voltage nonlinear resistors

ABSTRACT

A VOLTAGE NONLINEAR RESISTOR HAS A SINTERED WAFER OF, AS A MAJOR PART, ZINC OXIDE AND, AS AN ADDITIVE, IRON OXIDE AND ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF NICKEL OXIDE, TITANIUM OXIDE, MAGNESIUM OXIDE, MANGANESE OXIDE, CHROMIUM OXIDE, COBALT OXIDE, BARIUM OXIDE, STRONTIUM OXIDE, BERYLLIUM OXIDE, CADMIUM OXIDE AND LEAD FLUORIDE. A SILVER PAINT ELECTRODE IS APPLIED TO AT LEAST ONE OF THE OPPOSITE SURFACES OF SAID SINTERED WAFER.

Oct. 16, 1973 TAKESHI M ASUYAMA ETAL VOLTAGE-NONLINEAR RES I STOHS Filed June 22, 1970 BYMMMM ATTORNEYS UnitedStates Patent Office 3,766,098 Patented Oct. 16, 1973 3,766,098 VOLTAGE-NNLHNEAR RESISTORS Takeshi Masuyama and Tsuyoslii Nislii, Osaka, and Toshioki Amemiya, Tokyo, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan Filed June 22, 1970, Ser. No. 48,333

Claims priority, application Japan, July 2, 1969,

44/53,638, 44/53,641; July 4, 1969, 44/54,238,

i4/54,239, 44/54,240; Sept. 1, 1969, t4/70,424,

Int. Cl. Hlllb 1 06 U.S. Cl. 252-519 5 Claims ABSTRACT 0F THE DISCLOSURE A voltage nonlinear resistor has a sintered wafer of, as a major part, zinc oxide and, as an additive, iron oxide and one member selected from the group consisting of nickel oxide, titanium oxide, magnesium oxide, manganese oxide, chromium oxide, cobalt oxide, barium oxide, strontium oxide, beryllium oxide, cadmium oxide and lead uoride. A silver paint electrode is applied to at least one of the opposite surfaces of said sintered wafer.

V n f=(-) where V is the voltage across the resistor, I is the current ilowing through the resistor, C is a constant equivalent to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:

n: IOgioUz/Ii 1Ogm (V2/V1) Where V1 and V2 are the voltages at given currents I, and I2, respectively. Conveniently, I1 and I2 are 10 ma. and 100 ma., respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.

Silicon carbide varistors are most widely used as voltage-nonlinear resistors and are manufactured by mixing tine particles of silicon carbide with water, ceramic binder and/or conductive material such as graphite or metal powder, pressing the mixture in a mold to the desired shape, and then drying and tiring the pressed body in air or a non-oxidizing atmosphere. Silicon carbide varistors with conductive materials are characterized by a low electric resistance, i.e. a low value of C and a low value of n whereas silicon carbide varistors without conductive materials have a high electric resistance, i.e. a high value of C and a high value of n. It has been difcult to manufacture silicon carbode varistors characterized by a high n and a low C. For example, silicon canbide varistors, selenium oxide or cuprous oxide rectiexhibit n-values from 2.5 to 3.3 and C-values from 6 to 13 at a given current of 100 ma., and silicon carbide varistors without graphite have n-values from 4 to 7 and C-values from 30 to 800 at a given current of 1 ma. with respect to a given size of varistor, e.g. 30 mm. in diameter and 1 mm. in thickness.

Conventional rectiers comprising selenium oxide or cuprous oxide have an n-value less than 3 and a C-value of 5 to 10 at a given current of 100 ma. with respect to a specimen 20 mm. in diameter. In this case, the thickness of sample does not affect the C-value.

A germanium or silicon p-n junction resistor has an extremely high value of n but its C-value is constant, e.g. of the order of 0.7 to 0.3 at a given current of ma. because its diffusion voltage in the V-I characteristic is constant and cannot lbe changed very greatly. It is necessary for obtaining a desirable C-value to combine several diodes in series and/or in parallel. Another disadvantage of such diodes is the complicated steps involved in their manufacture, with resultant high cost. As a practical matter, the use of diode resistors is not widespread at the present in View of their high cost even through they may have a high value of n.

An object of this invention is to provide a voltagenonlinear resistor having a high value of n and a low value of C.

A further object of this invention is to provide a voltage-nonlinear resistor capable of being made by a simple manufacturing method which results in a low cost.

A further object of this invention is to provide a voltage-nonlinear resistor characterized by a high stability with respect to temperature, humidity and electric load.

Another object 0f this invention is to provide a volttage-nonlinear resistor, the C-value of which can be controlled.

These objects are achieved by providing a voltagenonlinear resistor which has a sintered Wafer of, as a major part, zinc oxide and, as an additive, iron oxide and one member selected from the group consisting of nickel oxide, titanium oxide, magnesium oxide, manganese oxide, chromium oxide, cobalt oxide, barium oxide, beryllium oxide, cadmium oxide and lead iluoride. Electrodes are applied to the opposite surfaces of said sintered wafer, at least one of which electrodes being a silver paint electrode. Leads are then soldered to the electrodes.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single ligure is a partly cross-sectional view through a voltage-nonlinear resistor in accordance with the invention.

Before proceeding with a detailed description of the voltage-nonlinear resistors contemplated by the invention, their construction -will be described with reference to the aforesaid figure of the drawing wherein reference character 10 designates, as a whole, a voltage-nonlinear resistor having, as its active element, a sintered wafer 1 of electrically conductive ceramic material according to the present invention.

Sintered wafer 1 is prepared in a manner hereinafter set forth, and is provided With a pair of electrodes 2 and 3 having specified compositions and applied in a suitable manner hereinafter set forth, on two opposite surfaces of the wafer.

The wafer 1 is a sintered plate having any one of various shapes such as circular, square, rectangular, etc. Wire leads S and 6 are attached conductively to the electrodes 2 Iand 3, respectively, -by a connection means 4 (solder or the like).

According to the present invention, sintered Wafer 1 consists essentially of, as a major part, zinc oxide and as an additive, iron oxide and one member selected from the group consisting of nickel oxide, titanium oxide, magnesium oxide, manganese oxide, chromium oxide, cobalt oxide, barium oxide, strontium oxide, beryllium oxide,

cadmium oxide and lead fluoride and at least one of electrodes 2 and 3 consists of a silver paint electrode.

Table l shows operable and optimal compositions of sintered body 1 for producing a voltage-nonlinear resistor having an n-value higher than 7 and a high stability with respect to temperature, humidity and electric load.

Table 2 shows operable and optimal compositions of silver electrodes 2 and 3 after heating for curing in order to produce the novel voltage-nonlinear resistors in accordance with the invention.

Table 3 shows optimal combinations of sintered body 1 and silver electrodes 2 and 3 for producing voltagenonlinear resistors having a C-value lower than 6 at a given current of 100 ma., an n-value higher than 20 and a high stability with respect to temperature, humidity and electric load.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials in having the compositions defined in Table 1 are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg/cm.2 to 1000 kg./cm.2. The pressed bodies are sintered in air at 1250 C. to 1450 C. for l to 3 hours, and then furnace-cooled to room temperature (about to about 30 C.). The pressed bodies are preferably sintered in a non-oxidizing atmosphere such as nitrogen and argon when it is desired to reduce the electrical resistivity. The electrical resistivity also can be reduced by air-quenching from the sintering temperature to room temperature even when the pressed bodies are lred in air.

The iron oxide, nickel oxide, titanium oxide, magnesium oxide, manganese oxide, chromium oxide, cobalt oxide, barium oxide, strontium oxide, beryllium oxide and cadmium oxide which are added to Zinc oxide as an additive may be in the form of metal, or carbonate or in any other form which in firing at the temperature employed will be converted to oxide.

The mixtures may be preliminarily calcined at 700 t0 l000 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed may `be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body have the opposite surfaces lapped by abrasive powder such as silicon carbide having a particle size of 300 mesh to 1500 mesh.

The sintered bodies are coated on the opposite surfaces thereof by a silver electrode paint in a per se conventional manner such as by a spray method, screen printing method or brushing method. It is necessary that the silver electrode paint have a solid ingredient composition as defined in Tables 2 and 3 after it is fired at 400 C. to 850 C. in air. Solid ingredients having compositions dened in Table 2 and 3 can be prepared in a per se conventional manner by mixing commercially available powders with organic resin such as epoxy, vinyl and phenol resin in an organic solvent such as butyl acetate, toluene or the like so as to produce silver electrode paints.

The silver powder may be in the form of metallic silver, or in the form of silver carbonate or silver oxide, or in any other form which in ring at the temperatures employed will be converted to metallic silver. Therefore, the term silver as used throughout this specification and the claims apended hereto in connection with the silver composition before it is fired, is meant to include silver in any form which in firing will be converted to metallic silver. The viscosity of the resultant silver electrode paints can be controlled by the amounts of resin and solvent. Particle sizes of solid ingredients also are required to be in the range of 0.1M to 5,u.

Lead wires can be applied to the silver electrodes in a per se conventional manner by using conventional solder having a low melting point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent for connecting the lead wires to the silver electrodes.

Voltage-nonlinear resistors according to this invention have a high stability with respect to temperature and in a load life test, which is carried out at 70 C. at a rating power for 500 hours. The n-value and C-value do not change greatly after heating cycles and the load life test. It is preferable for achieving a high stability to humidity that the resultant voltage-nonlinear resistors be embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se well known manner.

According to the invention, it has been discovered that the method of curing the applied silver electrode paint has a great effect on the n-value of the resultant voltage-nonlinear resistor. The n-value will not be optimal when the applied silver electrode paint is heated in a non-oxidizing atmosphere such as nitrogen and hydrogen for curing. It is necessary for obtaining a high n-value that the applied silver electrode paint be cured by heating in an oxidizing atmosphere such as air 'and oxygen.

Silver electrodes prepared by any other method than by silver painting result in a poor n-value. For example, the sintered body does not become a voltage-nonlinear resistor when it is provided with silver electrodes on the opposite surfaces by electroless plating or electrolytic plating in a conventional manner. Silver electrodes prepared by vacuum evaporation or chemical deposition result in an n-value less than 3.

The following examples are given as illustrative of the presently-preferred method of proceeding according to the present invention; however, it is not intended that the scope of said invention be limited to the specific examples.

EXAMPLE 1 Respective starting materials according to Table 4 are mixed in a wet mill for 5 hours.

The mixture is dried and pressed in a mold into a disc of 13 mm. diameter and 2.5 mm. thickness at a pressure of 340 kg./cm.2.

The pressed body is sintered in air at 1350 C. for l hour, and then quenched to room temperature (about 15 to about 30 C.). 'Ihe sintered disc has the opposite surfaces thereof lapped by silicon carbide with a particle size of 600 mesh. The resulting sintered disc has a size of 10 mm. diameter and 1.5 mm. thickness. The sintered disc is coated on the opposite surfaces thereof with a silver electrode paint by a conventional brushing method. The silver electrode paint employed has a solid ingredient composition according to Table 5 and is prepared by mixing these ingredients with vinyl-resin in amyl acetate. The coated disc is red at 800 C. for 30 minutes in air.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the resultant resistors are shown in Table 4.

EXAMPLE 2 Sintered discs having compositions listed in Table 6 are prepared in the same manner as those in Example l. Each sintered disc has a size of l0 mm. diameter and 1.5 mm. thickness. The sintered disc is coated on one surface thereof with a silver electrode paint having a composition listed in Table 7 in the same manner as that in Example 1 and is then fired at 500 C. in air. Another surface is provided with a spray metallized film of aluminum by a per as well known technique.

Lead wires are attached to the silver electrode and the aluminum electrode by means of conductive silver paint. The electric characteristics of the resultant resistors measured in the easy current ow direction are shown in Table 6. t

EXAMPLE 3 A sintered disc having a composition of 99.0 mol. percent of zinc oxide, 0.5 mol. percent of iron oxide and 0.5

mol. percent of beryllium oxide is prepared in the same TABLE 1 manner as that in Example 1. The sintered disc has a size of 10 mm. diameter and 1.5 mm. thickness. Various Operable composition 0f sintered bdy (m01- Percent) silver electrode paints are applied to opposite surfaces Zno Additives of the sintered disc and fired in air at a temperature listed y .i 99.9 to 80.0 0.05 t510.0 Fe 0.05 to 10.0 in Table 8. The silver electrode paints have solid ingredi- N10. z a ent compositions shown in Table 8 and are prepared by 99-9 t0 80-0 m5 if; 10-0 11820310115 t0 100 mixing 100 weight parts of said solid ingredient composi- 99.9 t5 80.0 0.05 :010.0 Feio., 0.05 to 10.0

M00 tions with 1 to 20 weight parts of epoxy resin in 20 to 40 00 0 to 800 0 05 to 100 F3203, 0 05 to m0 weight parts of butyl alcohol. The resultant voltage-non- 10 Mno. linear resistors have desirable C-values and n-values as 999 t0 80'@ -05rt2 10-0 F9203 0'05 W10-0 indicated in Table 8. It will be readily understood that 99.0 to 80.0 0.05 to 10.0 F920., 0.05 to 10.0

C00. the electrode comPqSltons have a great effect on. the 99.9 to 80.0 0.05 to 10.0 F920.. 0.05 to 10.0 electrical characteristics of the resultant voltage-nonlinear BaO. resistors 99.9 to 80.0 0.0s5 g 10.0 F520., 0.05 to 10.0 EXAMPLE 4 0055 1:8100 F920., 0.05 to 10.0 n n l u e A sintered disc having a composition of 99.0 mol. per- 0.0055100 Feio., 0.05 to 10.0 cent of zinc oxide, 0.5 mol. percent oi? iron oxide and 0.5 990 to 800 0 05 000100 F0200 0 05 to 100 mol. percent of magnesium oxide is prepared in the PbF.. same manner as that in Example 1. The sintered disc has Optimal Composition oislntered body (mOL pement) a size of 10 mm. diameter and 1.5 mm. thickness. Various silver electrode paints are applied to one surface of ggg tg gg "I gi 3:0) 85% ffg ,10158; the sintered disc and tired in air at a temperature listed 5 E0 083,002() 0 0 gc. .0. ...0.e23.0.0n. in Table 9. T he silver electrode paints have solid ingredi 900 to 040 0 1 to 3 0 F9203; 0 1 to 3 0 Choa ent compositions shown in Table 9 and are prepared by 25 99.8 to 94.0 0.1 to 3.0 F9203, 0.1 to 3.0 Coo. mixing 100 Weight parts of said solid ingredient com- 332g 2g gig I gj 2g 23 lgg'gj 18 :Isarag positions with 1 to 20 weight parts of epoxy resin in 20 99.8 to 94.0 0.1 to 3.0 Feiosl 0.1 to 3.0 BeO. to 40 weight parts of butyl alcohol. Another surface is ggg g 31% 'gg gig tig gg ggg; provided with a nickel electrode by an electroless plating TABLE 2 Operable composition of electrode (wt. percent) Ag Pbo Bno. Sio. Bio, Coo Mno 70.0 to 99.0 1.0 to 20.0 0 to 10 0 to 10 0 to 6.0 70.0 99. 1.0m 20.0 oto 0to10 0015.0. 70.0 0 to i0 0 to i0- 70.0 1.00530 0..-.- 0toi0 0to10 Optimal composition oi electrode (wt. percent) 75.0 to 95.0 2.95 to 15.0 1.0 to 5.0 1.0 to 5.0 0.05 to a 0 75.0 to .0 1.0 .05 3 0 75.0 to .0 75.0 .0

TABLE 3 [Optimal combinations of sintered body and electrode] Composition o Sintered Body (mol. percent) Composition of Electrode (wt. percent) ZnO Additive Ag B120: S102 B203 C00 80.0 t0 90.0.-.- 7. 0 to 14. 0.--.- 1. 0 t0 4. 0..-.- 1. 0 to 4. 0--.-- 1. 0 to 4. 0.

method. The resultant voltage-nonlinear resistors have desirable C-values and nvalues as indicated in Table 9.

EXAMPLE 5 The resistors of Example 1 and Example 2 are tested according to the methods used in testing electronic component parts. The load life test is carried out at 70 C. ambient temperature at 1 and 2 watts rating power for 500 hours. The heating cycle test is carried out by repeating 5 times a cycle in which said resistors are kept at 85 C. ambient temperature for 30 minutes, cooled rapidly to 20 C. and then kept at such temperature for 30 minutes. After heating cycle and load life tests, the change rates in the C-value and n-value are as shown in Table 10.

EXAMPLE 6 Zinc oxide sintered discs having additives in the amounts listed in Table 11 are prepared in the same manner as that in Example 1. Silver electrode paints having a solid ingredient compositions listed in Table 11 are coated on opposite surfaces of the respective sintered discs in the same manner as that in Example 1 and are red at 800 C. in air. The electrical characteristics of the resultant voltage-nonlinear resistors are shown in Table 1l.

TABLE 4 Electric characteristics Starting materials (mol. percent) of resultant resistors C (at a given current of ZnO Additives ma.) 'n

9918)-- FezOa 0.05, NO 0.05.

0. 4 7. 5 99.9- F0203 0.05, T102 0 0 2. 5 7. 0 99.8- F0203 0.1, T101 0 3. 8 10. 0 90.9. F6203 0.1, T1 4. 2 11. 4 94.0. F8203 3.0, Tl 4. 7 10. 8 80.0 F8203 10.0, T102 6. 4 7. 2 99.9- F8203 0.05, Mgo 0 05 2. 8 7. 8 99.8- F6203 0.1, Mg 3. 9 12. 0 96.9. F9202 0.1, MgO 3 4. 5 15. 4 94.0. F8203 3.0, MgO 3 5. 7 11. 8 80.0. F0203 10.0, Mg() 7. 2 8. 5 99.9- F6203 0.05, Mn0 0 3. 4 7. 1. 99.8 F8203 0.1, Milo 4. 5 10. 4 90.9- F8203 0.1, M 5. 2 11. 6 94.0 F8203 3.0, M110 5. 3 11. 5 80.0, F0203 10.0, M110 7. 5 7. 0 99.9. F9203 0.05, CrzOs 2.8 7. 4 99.8 F6203 0.1, 01'203 0 1 4. 5 10. 3 96.9 F6203 0.1, Cl203 3.0- 5. 1 11. 4 94.0 F6203 3.0, 01'203 3.0 5. S 11. 2 80.0 F6203 10.0, 01'203 10.0 0. 4 7. 9 99.9 Fe2O3 0.05, C00 0.05 3. 8 8. 5 99.8 F820; 0.1, CO0 0.1 4. 5 14. 3

TABLE 6Continued Electric characteristics oi Starting materials (mol. percent) resultant resistors C (at a given current of TABLE 4-Continued Electric characteristics of resultantl resistors C (at a Starting materials (mol. percent) given current of Additives 100 ma.)

ZnO

TABLE 7 Composition of silver electrode (Wt. percent) PbO SiO: B203 The characteristics oi the resultant resistors Firing tem- C (at a perature of given curelectrode rent ci MnO C.) 100 ma.)

TABLE 8 B203 COO SlOz 5.7.&&5.5.7.&& 111111111 O BlzOg Composition of electrode (wt. percent) The characteristics of the resultant resistors Firing temperature oi C (at a given electrode current of MnO C.) 100 ma.)

TABLE 9 SiO;

5. A voltage-nonlinear resistor as claimed in claim 1, wherein said silver electrode has a composition of 75.0

Changes rates after 2-Watt life n test (Percent) Heating cycle test TABLE 11 Characteristics of resultant resistors TABLE 10 de, the opposite surfaces of the Composition of electrode (wt. percent) PbO l-watt life test Z-Watt lie test Change rates in the C-value and 'lt-value after test Sintered body additives (mol. percent) Example 1...---{ Example --lf:

Sample 11100010100 0.n0.&3.3.0.3.0.3.1

1. A voltage-nonlinear resistor comprising a sintere oxide and 0.05 to 10.0 mole percent of one mem lected from the group consisting of titanium oxide, manof the wafer, and a silver paint electrode on at least one te surfaces of said sintered Wafer.

-nonlinear resistor as claimed in claim 1,

ercent of iron oxide and 0.1 to 3.0 mole percent of oxide, manganese oxide, barium oxide, strontium -nonlinear resistor as claimed in claim 1, wherein a silver electrode is on both opposite su 4. A voltage-nonlinear resistor as claimed in claim 1, 75 wherein said silver electrode has a composition of 70.0 252 520, 521 

